1. Field of the Invention
The present invention relates to a lead storage battery, and more specifically to a thin sealed type lead storage battery which is constituted by holding battery cells in a battery casing formed by superposing a pair of split cases.
2. Description of the Prior Art
Thin, sealed-type lead storage batteries of the above-mentioned type have been known as disclosed in, for example, Japanese Patent Laid-Open No. 33828/1976, Japanese Utility Model Laid-Open No. 159162/1983, and Japanese Utility Model Laid-Open No. 157969/1983. Lead storage batteries disclosed in these publications are briefly described below.
First, according to the lead storage battery disclosed in Japanese Patent Laid-Open No. 33828/1976, collectors and active material are arranged in a pair of split cases which are superposed under the condition in which separators are held in the active material. In this known lead storage battery, however, external terminals are protruded from the outer surfaces of split cases to take out the electric current from the collectors in the split cases to the units outside the battery. Therefore, the external terminals impose limitation on reducting the thickness of the battery, making it difficult to obtain batteries of desired thicknesses. In the lead storage batteries of this type, it is necessary to provide a safety valve for releasing gas. Thickness of the safety valve holding portion serves as another significant factor which imposes limitation on reducing the thickness of the batteries.
In the lead storage battery disclosed in Japanese Utility Model Laid-Open No. 159162/1983, the positive and negative electrode formed by adhering electro-collecting boards onto one side of the active material layer are contained in a battery casing made of a resin via separators, holes are formed in the wall of the battery casing opposed to the electro-collecting boards, through holes are formed in end portions of the battery casing, rivet fittings that serve as external terminals of the positive and negative electrode are attached to the through holes, and portions of the electro-collecting boards facing the holes and rivet fittings are connected via lead wires that run on the surface of the battery casing. However, the thus constructed lead storage battery has problems as described below. First, lead wires are connected by soldering to the electro-collecting boards or the rivet fittings. Therefore, the electro-collecting boards, lead wires, rivet fittings and the battery casing are damaged by the heat of soldering. Further, holes of the battery casing from where the lead wires are taken out, are sealed with a thermosetting resin such as an epoxy resin or the like. However, the holes are formed so shallow that there is no space for charging the epoxy resin in amounts sufficient to accomplish complete sealing. Therefore, the electrolyte in the battery casing often leaks from the sealed portions. Moreover, the rivet fittings that serve as external terminals assume the shape of a rod, and it is difficult in manufacturing the batteries to always secure the rod-like rivet fittings perpendicular to the plane of the battery casing. If the rivet fittings are tilted even by the slightest angle, complete electric contact is not obtained relative to the receiving terminals on the side of the electric equipment.
With the lead storage battery disclosed in Japanese Utility Model Laid-Open No. 157969/1983, electro-collecting boards composed of lead or a lead alloy are mounted with the battery casing, and the electrode plates consisting of synthetic resin nets carrying an active material are closely adhered onto the electro-collecting boards, to realize a laminated thin battery structure having a cross section consisting of battery casing, electro-collecting board, cathode layer, separator, positive electrode layer, electro-collecting board and battery casing. However, this lead storage battery has problems as described below. First, in the conventional batteries, the battery casing is warped by the contraction of resin after molding, marring the appearance, and causing the output of the battery casing to decrease. Therefore, discharge capacity of the battery decreases greatly through charge-discharge cycles of the battery, and the life of the battery is considerably shortened. Further, discharge capacity reduction varies greatly.